The principles are clear, the applications are more difficult

The Catholic Church has reflected long and hard on the use of violence, and the theory of just war has evolved as a way of laying out the conditions under which a way may be justified morally. It could be argued that no other system of analysis has come close in defining such an approach to war.

The just war theory posits a number of key conditions. A war must be defensive in nature and be undertaken by a legitimate authority. The damage sustained 'must be lasting, grave and certain' in nature so as to justify a military response. War must be the last resort in addressing such damage. There must be a serious prospect of success, and the principle of proportionality must be observed in the waging of war.

While the principles are relatively clear, applying them to particular contexts leaves room for different judgments and opinions. I would argue that the intervention in Iraq, and to a lesser extent, in Syria, meet the conditions of just war theory. The case against ISIS in terms of it being an aggressive force inflicting lasting, grave and certain damage is compelling.

Millions of Iraqis and Syrians have been displaced and there is widespread hunger. There has been a deliberate policy of ethnic and religious cleansing: ancient Christian communities have been expelled, and the Yazhidi minority, who pose no threat to any group by virtue of their small numbers and isolation, have been particularly targeted for extermination. There has also been systemic cultural cleansing of many historic sites such as the Green Church in Tikrit, the tomb of Jonah and Shia shrines and mosques.

Women have been targeted, with rape used as weapon of terror and women, and girls, sold into sexual slavery and forced marriages. There has been forced genital circumcision of both males and females. Crucifixions and beheadings have been employed as a method of control, such as the beheading of the prominent human rights lawyer, Samira Salih al-Nuaim. Captured prisoners of war have been summarily executed. Millions of Shia, Christians, Kurds, and others, are in harm's way in what is potentially a humanitarian disaster on a scale similar to that of Kampuchea's killing fields or the butchery of Rwanda. The humanitarian argument for military intervention is compelling.

Moreover, the genuine danger to world peace is also a persuasive argument for intervention: a Sunni-Shia bloodbath could plunge the Middle East into war as Saudi Arabia and Iran are drawn in. An ISIS advance towards the borders of Israel would carry enormous risk. And just as ISIS has become a magnet for thousands of foreign fighters, so too, it is not unreasonable to see it becoming a base for attacks on civilian targets around the world. Indeed, ISIS' own rhetoric calls for waging such terrorism. These reasons perhaps also answer the question as why military intervention is justified in this case when there may be no intervention in other parts of the world in which there is violence and bloodshed.

The legitimate government of Iraq has asked for military support (the issue of the Syrian government under Assad, which has major questions of legitimacy, is a reason why intervention in Syria is less clear from the perspective of just war theory). The range of nations prepared to join the campaign also adds legitimacy to the case for intervention, as an international effort is not tied to the interest of any one nation.

There appears to be a determination on the part of the combatant nations to minimise the potential for civilian casualties, thus meeting the test of proportionality. The execution of four westerners by beheading, along with its general policy of terror and its violent ideology, would suggest that ISIS has no intention of negotiating, and indeed welcomes war, thus meeting the test of last resort. The possibility of success in military action against ISIS appears to be most problematic in terms of just war theory, at least in terms of neutralising ISIS. In averting the fall of the Kurds or the Shia, and of containing ISIS in the short term, and of saving hundreds of thousands of lives by doing so, the case is more easily made that military action might be worthwhile, especially as there seems to be no alternative.

The responsibility that comes with military intervention is immense. It is easy for fear, hatred and prejudice to rise to the surface. In this case of conflict with ISIS in Iraq it is vital that we understand that most of its victims are Muslim.

In an understandable concern for security here at home we must be vigilant in opposing any vilification or discrimination against our fellow citizens who are Muslim. Islam, itself, is very broad movement of over a billion believers and contains diversity just as Christianity does: there are fundamentalists, moderates, conservatives and liberals; there are also different sects and communities.

We need to remind ourselves that for a significant period in history the Islamic world was the home to the best in science and learning, which European society came to owe much to. Nor should we forget that much of the Islamic world is still emerging from the demoralising experience of Western colonial rule, explaining in part the frustration and anger that sometimes is associated with Islam.

Nor should we blind ourselves to the fact that profoundly negative and hostile forces can rise in the name of religion or nationalism or ideals such as liberty or equality. This past century has witnessed numerous examples of extreme ideologies coming to power with disastrous results for the world. To intervene in Iraq carries real risks; to turn aside, in my view, carries substantially greater risks.

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